This invention relates to a DC AC converter for igniting and supplying a low-pressure discharge lamp. The converter is provided with two input terminals intended to be connected to a d.c. voltage source, the input terminals being connected together by means of a series arrangement of a semiconductor switching element having a control electrode, a first coil and a parallel circuit having the discharge lamp incorporated in one of its branches. The control electrode and a main electrode of the controlled semiconductor switching element are connected together by means of a secondary winding of a transformer whose primary winding forms part of the parallel circuit. A starter circuit is incorporated in the converter.
A DC AC converter of this type is known from U.S. Pat. No. 4,415,838 to applicant.
This converter energizes, for example, an electrodeless low-pressure discharge lamp having an operating frequency of at least 0.5 MHz or a compact low-pressure mercury discharge lamp. It has been found that the converter has a high efficiency.
In order to start the converter, a starter circuit is incorporated in the control circuit of a semiconductor switching element consisting, inter alia, of a series arrangement of a resistor and a capacitor connected to the input terminals of the converter. Furthermore, the starter circuit has a branch comprising a bidirectional breakdown element (diac) which is connected at one end to a junction between the said resistor and capacitor and at the other end to the control electrode of the semiconductor switch.
It has been found that the use of the capacitor and the said breakdown element in the starter circuit often gives rise to an increased level of radio interference. In fact, the said capacitor is charged over and over again via the resistor up to the breakdown level of the diac, whereafter a start pulse in the operating converter is supplied. Furthermore, it has been found that the breakdown current of the diac at higher temperature increases over the course of the operating time. For reliable starting a circuit is to be designed for a fairly high breakdown current which also flows through the said resistor. In that case extra thermal losses occur. This is a drawback, particularly if the converter and the lamp are integrated to form a compact unit.